If you have an Introit sender this note might help. I got this from the Introit company to calibrate My sender:
BTW the sender should be about one inch off the bottom.

Ethanol is a big complication for capacitance-based senders, of which our is one. Fuel with 10% ethanol has twice the capacitance of fuel without ethanol. That means if a sender were calibrated for fuel with 10% ethanol but was used in fuel without ethanol, it would only read 1/2 at full. And the percent ethanol can be anywhere between 0 and 10%-- we have about 5% in the Daytona Beach area last I measured.
To correct for that, we have a Full Detection stinger toward the top of the sender. The scheme the sender uses is:
--Sender powers up with a reading below 1/2. This "arms" the Full Detection

--Sender power up with armed Full Detection and the Full Detection stinger in fuel. It assumes the tank has been filled and sets that amount of capacitance as the current Full cal. Each tankful would repeat that process.

I'm guessing that you may have powered up with the tank not yet full but will with fuel on the Full Detection stinger, as part of your testing by filling, and that created a false Full Detection. So for this tankful you'll read above Full until you get down to that level, and then on down. When you refill with an actual full tank, it will correct itself, assuming its been powered up with a reading below 1/2 before that (the reason for the arming at 1/2 or lower is to insure that fuel on the FD stinger is not just due to a tilted tank).

After looking at your picture a little closer, I really think that you have one of those with the magnet inside. Search on this site, they can be repaired. The magnet has a way of seperating from its position.

Thanks for the answer ,
yes it is the type with a magnet inside ,and yes it had come off , so I have glued it back on to the float.
With sender out of the tank , I have done a resistance check , and it rises and falls as per the location of the float .
You are correct , I have not filled the tank with fuel , and now it only has 1/3 in it ,but the gauge only just read off empty.

I thought these senders just changed their resistance as per the position of the float .

If the sender is 2" off the bottom of the tank it will never register fuel on the gauge until the float starts to lift above 2",unless I am missing something .

You're right. The note I got for my unit is just for that unit. I think your diagnosis is correct. Maybe someone with that kind of unit can chime in and get you up to speed. I hope you get your answer.